[lord12]

how can u tell if a cooridination compound of coordination number 4 is square planar or tetrahedral?
for example
CD(CO)4 3+? knowing it has 1 upaired spin

[Dan]

Presumably you mean Cd, not CD.

There aren't any hard and fast rules to tell, it's mainly chemical intuition, but there are some things that suggest square planar.

eg. d⁸ electron config at the metal centre
eg. 16 electrons rather than 18 in organometallics.

have a read of this http://science.kennesaw.edu/~mhermes/cisplat/cisplat06.htm

[AWK]

CD(CO)4 3+? knowing it has 1 upaired spin

?!
cadmium +3 probably Co (and it forms carbonyl compounds)

[Yggdrasil]

If the metal in a transition metal complex with coordination number of four is low spin, it will be square planar.  If the metal is high spin, it will be tetrahedral.  To figure out if the metal is high or low spin you have to look at both the identity of the metal and the ligands attached to it.  Here are some general rules:

Any 5d transition metals will be low spin. 
4d transition metals will prefer low spin unless the ligand is low in the spectrochemical series (i.e. a halogen). 
3d transition metals will prefer high spin unless the ligand is high in the spectrochemical series (i.e. a ligand capable of pi backbonding).